Hell hath no fury like a Demi Lovato Opens a New Window. scorned… or a Demi Lovato in an intense boxing session!

On Tuesday, March 5, the singer posted an Instagram video Opens a New Window. bragging about accidentally punching out the front tooth of her trainer, Jay Glazer. “Holy shit I literally knocked @jayglazer’s tooth out during training this morning – while he was wearing a mouth piece!!!!” she captioned the post. “Hahahahaha sorry (not sorry) Jay!!! So coach, when’s my first fight?! .” She hashtagged the name of the elite West Hollywood training facility, #unbreakableperformance.

Glazer was up for celebrating the unlikely feat too. In the video, the boxing trainer rings the gym’s bell to gather everyone’s attention. Lovato stands next to him with her boxing gloves still on. “Ring the bell, ring the bell, ring the bell, ring the bell” he yelled out while holding up his tooth and smiling. “Demi knocked my tooth out. Demi knocked my tooth out!”

Out of the frame, someone else held up her left arm in victory while another person in the background yelled out, “Winner!”

When Lovato, 26, laughingly apologized and hugged the pro, Glazer responded, laughing, “She’s not sorry! She’s not sorry at all!” He then confirmed that his mouthpiece was in when the incident occurred.

The “Sober” singer has long worked out at the Unbreakable Performance Center, which also trains its clients in mixed martial arts. And her fitness regimen there is intense Opens a New Window. . “The MMA part has definitely resonated the best with her,” Glazer told Us in 2016. “She actually hits and kicks incredibly hard, people here are kind of stunned when they look in the cage to see who’s cracking the pads that hard and see it’s her. Gets a good laugh from us.”

Hilariously, she once impressed Rocky himself. “Sylvester Stallone came in and heard someone hitting pads and couldn’t believe it was her,” added Lovato’s coach. “Not a bad plug to get from Rocky!”

Her technique developed after years of hard-hitting, high-performance training. “We actually train Demi the same way we would one of our professional fighters or pro athletes,” said the pro. “In a typical day she will do at least one form of combative training — boxing, Muay Thai and jiu-jitsu — and then a performance workout with the same coaches who train our NFL [clients].”

Since leaving rehab following her near-fatal July overdose Opens a New Window. , Lovato has been dedicating more time than usual to her workout sessions, for both her physical and mental health. “Working out is cleansing for her mind, body and soul,” a source told Us Opens a New Window. in November 2018. “It keeps her on track, focused and feeling good.”